What happens during an emissions test and what you need to know before you go
An emissions test measures how much pollution your vehicle's exhaust produces. A technician will connect a probe to your tailpipe, run your engine at different speeds, and record the results. Your vehicle passes if the emissions fall below the limits set by your state — those limits vary by state, vehicle age, and vehicle type. The test itself takes 20 to 45 minutes and costs between $30 and $75 depending on where you live.
Before you arrive at the testing station, your vehicle should be in reasonable mechanical condition. A vehicle with a check engine light, a broken oxygen sensor, or a clogged air filter will almost certainly fail. The good news is that many of these problems are fixable before test day, and some are cheap to fix.
Key Takeaways
- Your vehicle must have a functioning oxygen sensor, a clear air filter, and no active check engine light to have a realistic chance of passing.
- A pre-test inspection at a repair shop costs $50 to $150 but can catch problems before you pay for the official test.
- If your vehicle fails, you have a set window (usually 30 to 60 days) to make repairs and retest at a reduced or waived fee.
- Driving on the highway for 15 to 20 minutes before your test can help clear out carbon buildup and improve your results.
Check your vehicle's basic systems before test day
Start with the things you can see and feel. Make sure your air filter is clean — a clogged air filter restricts oxygen to the engine and causes it to run rich, which means too much fuel and higher emissions. You can check this yourself by looking at the filter in your engine bay; if it looks gray or black instead of white, replace it. A new air filter costs $15 to $40 and takes five minutes to install.
Check your tire pressure. Underinflated tires increase rolling resistance, which makes your engine work harder and produce more emissions. Use a tire gauge to check all four tires and inflate them to the pressure listed on the driver's side door jamb, not the maximum pressure on the tire itself.
Look at your oil level using the dipstick. Low oil forces your engine to work harder and can cause it to burn oil, which raises emissions. Top it off if needed — use the grade recommended in your owner's manual. Old, dirty oil also makes emissions worse, so if your oil is dark brown or black, an oil change before the test is worth the $30 to $75 cost.
Get a pre-test inspection if your vehicle has warning signs
If your check engine light is on, do not go to the official test. The light means your vehicle's onboard diagnostic system has detected a problem, and you will fail. Take your vehicle to a repair shop and have them read the diagnostic code — this tells you what the problem is. Common codes that cause test failures include oxygen sensor faults, catalytic converter problems, and fuel system issues.
A pre-test inspection at a shop costs $50 to $150 and involves a mechanic running a diagnostic scan, checking your fuel pressure, and inspecting your exhaust system. This is money well spent if you are unsure whether your vehicle will pass. Many shops will waive the inspection fee if you have them do the repairs.
If your vehicle is older than 10 years or has high mileage, a pre-test inspection is especially useful. Older vehicles are more likely to have worn oxygen sensors, failing catalytic converters, or fuel system leaks — all of which cause test failures.
Common repairs that improve emissions test results
An oxygen sensor measures how much unburned fuel is in your exhaust and tells your engine's computer to adjust the fuel mixture. A faulty oxygen sensor is one of the most common reasons for test failure. Replacing one costs $150 to $300 depending on which sensor and which vehicle. There are usually two or four oxygen sensors on a vehicle; your mechanic will know which one is bad from the diagnostic code.
A catalytic converter cleans up the exhaust by converting harmful gases into less harmful ones. If it is failing or clogged, your emissions will be high. Catalytic converter replacement is expensive — $400 to $1,200 — but if your vehicle is relatively new and the converter is still under warranty, you may not have to pay. Check your warranty paperwork or call your vehicle's manufacturer.
A fuel system cleaning can help if your fuel injectors are clogged. This costs $100 to $200 and can sometimes improve emissions enough to pass a borderline test. It is worth trying before you replace expensive parts.
What to do in the days before your test
Drive your vehicle normally for at least a week before the test. This allows your engine's computer to run through its self-diagnostic cycle and clear out any temporary fault codes. If you have just fixed something, this driving period also helps the computer learn the new sensor readings.
About 15 to 20 minutes before your test appointment, drive on the highway at steady speeds between 45 and 65 miles per hour. This heats up your catalytic converter and burns off carbon buildup, which can lower your emissions readings. Avoid short trips or idling in traffic right before the test.
Make sure your fuel tank is at least half full. Some testing stations require this, and a full tank also helps your vehicle run more smoothly during the test. Fill up the day before if possible so the fuel has time to circulate through your system.
What happens if you fail and how to retest
If your vehicle fails, you will receive a written report showing which emissions exceeded the limits. Your state will give you a window — usually 30 to 60 days — to make repairs and retest. During this window, you can drive your vehicle legally even though it failed, as long as you have the failed test report with you.
Use the failure report to guide your repairs. If your nitrogen oxides (NOx) were high, the problem is usually ignition timing or a faulty sensor. If your hydrocarbons (HC) were high, you likely have a fuel system or ignition problem. If your carbon monoxide (CO) was high, your fuel mixture is too rich, which points to an oxygen sensor or fuel injector issue.
When you retest, many states waive the retest fee or charge a reduced fee. Bring your original failure report to the testing station. The retest uses the same procedure as the first test, so the same preparation steps explore.
Frequently Asked Questions
Can I pass an emissions test if my check engine light is on?
No. The check engine light means your vehicle's diagnostic system has detected a fault, and the testing station's equipment will read that same fault. You must have the problem diagnosed and repaired before you test. A diagnostic scan at a repair shop costs $50 to $100 and tells you exactly what needs to be fixed.
How long does an emissions test take?
The actual test takes 20 to 45 minutes depending on your vehicle type and your state's test procedure. Add time for check-in and paperwork. Plan for one hour total at the testing station. Some states test only at certain times of day, so check your local testing station's hours before you go.
What if I just had my vehicle serviced and it still fails?
Ask the repair shop what work they did and whether they cleared the diagnostic codes. Sometimes a vehicle needs to be driven for several days after service before the computer relearns and the codes clear. If the shop did not address the specific emissions problem, take your failure report back to them and ask them to diagnose why it failed.
Do I need to warm up my vehicle before the test?
Yes. Drive for 15 to 20 minutes at highway speeds before your test appointment. This warms up your catalytic converter and engine, which helps emissions readings. Avoid idling or short trips right before the test, as these do not warm up the catalytic converter enough.
What is the difference between a smog test and an emissions test?
These terms are used interchangeably in most states. Both measure tailpipe emissions and check your vehicle's onboard diagnostic system. Some states call it a smog test, others call it an emissions test or an inspection and maintenance (I/M) test. The procedure and standards are the same regardless of the name.